1. Field of the Invention
The present invention relates to a magnetic marker for pulse generation used in an article monitoring system or the like and a manufacturing method therefor.
2. Description of the Related Art
If magnetic markers (also called tags) used in an anti-shoplifting burglarproof system for commodities, for example, are provided on the outer surface of the commodities, they may possibly be removed maliciously. It is to be desired, therefore, that the markers should be previously loaded (for source tagging) into the commodities or packaging containers at the product production stage.
A low-coercivity material described in Jpn. Pat. Appln. No. 62-242319 or Jpn. Pat. Appln. KOKAI Publication No. 4-220800 is known as a prior art related to magnetic markers. Also known are a high-permeability, low-coercivity material described in U.S. Pat. No. 4,660,025 and strips or wires of which the magnetization curves exhibit major Barkhausen discontinuity.
Magnetic markers that are formed of these conventional magnetic materials have the following matters to be studied on their length. Thus, in order to generate high-level pulse signals that can be securely detected at a detection gate, the ratio “length/(cross-sectional area or diameter corresponding to cross-sectional area)” of the marker and the cross-sectional area have lower limits.
In the case of U.S. Pat. No. 4,660,025, for example, the antimagnetic field coefficient never exceeds 0.000125. This implies that the ratio “length/diameter corresponding to cross-sectional area” of the marker that uses an elongate magnetic substance such as a strip or wire cannot be lower than about 200. In the case of U.S. Pat. No. 3,747,086, on the other hand, the ratio “length/square root of diameter corresponding to cross-sectional area” exceeds about 200. Even if the aforesaid dimensional conditions provided by those individual prior arts are met, however, accurate detection requires a strip or wire length of 50 mm or more in the case where the passage width of the detection gate is 90 cm or more, in particular.
Described in Jpn. Pat. Appln. KOKAI Publication No. 4-195384, on the other hand, is a configuration such that the ratio “length/(cross-sectional area or diameter corresponding to cross-sectional area)” of a strip or wire can be lowered. More specifically, a longitudinal end portion of the strip or wire is provided with a soft magnetic foil that has a coercive force smaller than a coercive force of the strip or wire. This is expected to reduce antimagnetic fields that are generated in the longitudinal direction in the case where a strip or wire alone is used.
The antimagnetic fields are magnetic fields that are simultaneously generated in a magnetic material so as to restrain an external magnetic field (i.e., to prevent magnetization of the material) in a direction opposite to the direction of the external magnetic field in a manner such that magnetic poles (north pole on one side and south pole on the other side) are formed individually at the opposite ends of the magnetic material when the magnetic field is externally applied in a specific direction and magnetized, if the magnetic material is finite in the direction of the external magnetic field.
The aforesaid marker described in Jpn. Pat. Appln. KOKAI Publication No. 4-195384 has a problem that it requires a lot of manufacturing processes and entails increased cost, since it includes a number of components. According to this prior art, moreover, miniaturization of the marker is restricted in view of workability in working process for cutting the magnetic material and a process for lapping the low-coercivity material and the soft magnetic foil on each other, so that the marker is inevitably relatively conspicuous in appearance. Further, there are restrictions on the portion of an article on which the marker is provided. In the case where the marker is pasted on a curved surface, moreover, the respective contact portions of the soft magnetic foil and the strip or wire may be disengaged, and the properties of the marker may be worsened by deformation. Thus, the marker of this type is not always suited for source tagging.
Thus, in consideration of the manufacturability, external appearance, and miniaturization (reduction in width, in particular) of the marker, its stickability to curved surfaces, etc., this prior art has the same problems with the aforesaid marker of Jpn. Pat. Appln. KOKAI Publication No. 4-220800. In order to give an inactivating function to this marker of Jpn. Pat. Appln. KOKAI Publication No. 4-195384, moreover, a hard magnetic material should be provided along the strip or wire, so that the component configuration of the marker is further complicated, inevitably.
Accordingly, there has been a demand for magnetic markers that enjoy high productivity and low cost and are suited for source tagging.
Further, the magnetic materials described in Jpn. Pat. Appln. No. 62-242319, Jpn. Pat. Appln. KOKAI Publication No. 4-220800, U.S. Pat. No. 4,660,025 and the strips or wires of which the magnetization curves exhibit major Barkhausen discontinuity have a problem that the antimagnetic fields sharply increase as the ratio “length/(cross-sectional area or diameter corresponding to cross-sectional area)” lowers. Since the influence of the antimagnetic fields constitutes an obstacle to the magnetization of the strip or wire, meaning that the magnetic material cannot fulfill its essential functions. Thus, the ratio “length/(cross-sectional area or diameter corresponding to cross-sectional area)” has its lower limit.
The smaller the magnetic poles (intensity of magnetization) formed individually at the opposite ends of the magnetic material or the longer the distance between the two magnetic poles, the smaller the antimagnetic fields become. In the cases of wires and strips where an alternating field is applied in the longitudinal direction of the magnetic material and a signal based on magnetic inversion in the same direction is detected by means of a coil, therefore, the influence of the antimagnetic fields can be lessened by making the wire or strip long and slender. Thus, the higher the “length/(cross-sectional area or diameter corresponding to cross-sectional area)” is, the smaller the influence of the antimagnetic fields can be made.
In order to reduced the antimagnetic fields by means of the strip or wire alone, in other words, it is necessary only that its length be shortened without changing the lower limit of the ratio “length/(cross-sectional area or diameter corresponding to cross-sectional area)”. This implies that the cross-sectional area is also reduced. However, the level of a signal that can be detected by means of a coil in a detection gate is proportional to the product of the intensity of magnetization and cross-sectional area of the wire or strip and magnetic inversion speed. If the cross-sectional area is reduced in proportion to the length, therefore, a pulse signal cannot be discriminated from disturbance noise that is caught by the detection coil. Accordingly, the cross-sectional area also has a lower limit. On the other hand, the reduction of the cross-sectional area may possibly be compensated by increasing the intensity of magnetization of the material. However, this causes an increase of antimagnetic fields.
In the case of a magnetic marker that uses a conventional wire or strip, therefore, accurate discrimination from disturbance noise requires a magnetic marker length of at least 50 mm if the frontage (passage width) of the detection gate is 90 cm or more. Actually, however, there is a demand for small-sized wire-type markers with lengths of 40 mm or less that can be detected with high accuracy even if the passage width of the detection gate is 90 cm or more.
There is also a demand for markers that can be previously loaded (for source tagging) into commodities or packaging containers in the stage of their production so that an operator of a cash register or the like can inactivate the markers or cancel their pulse generating function without being conscious of the presence of the markers as he/she clears off the payment for the commodities. Since a marker is inactivated by placing a commodity having the marker therein on an inactivating apparatus or passing it over the inactivating apparatus, the markers are expected to be able to be inactivated without touching the inactivating apparatus.
Conventionally, there is a proposal to bring a marker having a low-coercivity material and a high-coercivity substantially into contact with the surface of an inactivating apparatus having a predetermined magnetic field pattern, thereby transferring the magnetic field pattern to the high-coercivity material, as is described in Jpn. Pat. Appln. No. 62-242319, for example. Once the high-coercivity material is polarized, in this case, the predetermined magnetic field pattern remains in it if it leaves the inactivating apparatus. Allowing the magnetization pattern to remain in this manner will be referred to as pattern polarization hereinafter.
A static bias magnetic field can be applied to the low-coercivity material of the magnetic marker by pattern polarization. This static bias magnetic field serves to prevent the low-coercivity material of the marker from undergoing magnetic inversion in an alternating field in the detection gate. Alternatively, the region of the low-coercivity material that undergoes magnetic inversion diminishes, so that a signal excited by the detection coil becomes extremely low. In consequence, the marker is inactivated. In this case, the magnetic field pattern of the inactivating apparatus must be transferred to the high-coercivity material, making it hard to inactivate the marker in a non-contact manner.
On the other hand, there is a proposal to expose a marker to a magnetic field that is formed by half-wave-rectifying a static magnetic field in one direction or alternating field, as is described in Jpn. Pat. Appln. KOKAI Publication No. 4-220800. In this case, a north or south pole can be left in the end portions of the high-coercivity material even after the marker is moved away from the magnetic field that is obtained by half-wave-rectifying the static magnetic field in one direction or alternating field. Accordingly, a desired static bias magnetic field can be applied without transferring the magnetic field pattern to the high-coercivity material. Thus, the marker can be inactivated in a non-contact manner.
The aforesaid technique described in Jpn. Pat. Appln. KOKAI Publication No. 4-220800 has a problem that the marker requires a lot of manufacturing processes and entails increased cost, since it includes a number of components. With use of the high-coercivity material described in this publication, moreover, miniaturization of the marker is restricted in view of workability in working process for cutting the material and a process for lapping on the low-coercivity material, so that the marker is inevitably relatively conspicuous in appearance. Further, there are restrictions on the portion of an article on which the marker is provided. In the case where the marker is pasted on a curved surface, moreover, the low-coercivity material may bend at the end portions of the high-coercivity material, thereby worsening in properties, owing to dislocation of the respective overlapping portions of the low-coercivity material and the high-coercivity material or difference in stiffness between the two materials. Thus, the marker of this type is not always suited for source tagging.
In order to solve these problems, the inventors hereof proposed a wire-type marker designed so that a magnetically switchable wire is covered by means of a magnetic casing for canceling, as is described in Jpn. Pat. Appln. KOKAI Publication No. 10-188151. Disclosed in connection with this prior art is an arrangement such that holes or notches are formed at given spaces in the magnetic casing for canceling, whereby a plurality of pairs of magnetic poles N and S can be polarized alternately. However, there is a demand for magnetic markers that enjoy higher productivity and lower cost and are more suited for source tagging.
Accordingly, a first object of this invention is to provide a small-sized magnetic marker with a simple construction that can be detected with high accuracy even in a gate having a wide passage. Further, a second object of this invention is to provide a magnetic marker that can be activated and inactivated in a non-contact manner.